Activation of the immediate early gene c-fos has been used as a marker of CNS neuronal activity. The construction of a transgenic mouse employing green fluorescent protein (GFP) as a reporter of Fos expression (Quintero et al SFN Abst. 1999: 2065) allows for cell morphology of activated neurons to be visualized in both live and fixed tissue. These mice were obtained (D. McMahon, U Kentucky) to assess the utility of neurochemically-induced GFP expression in the in vitro intact spinal cord. Mice (P4-20) were anesthetized before spinal cord isolation in aCSF. Following 3-5 hours, various drugs, including 5-HT, dopamine, strychnine, and bicuculline, were added for 2 hours and then cords were sectioned as fresh or fixed tissue. GFP was labeled with a polyclonal Rb antibody and compared to endogenous GFP fluorescence. Immunolabeled GFP was detectable in more neurons than observed with GFP fluorescence. Somata were clearly labeled and, in several neurons, processes were discernible. In all preparations, the subst...